Feat: Add Copy URL button and reusable toast notification system

Detail page now has a "Copy URL" button that copies the image's direct
file URL to the clipboard. A toast service (BehaviorSubject-backed,
auto-dismissing after 3s) confirms success or failure. ToastComponent
is registered at the app root and available to all future features.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/src/app/services/toast.service.spec.ts

@@ -0,0 +1,51 @@
+import { TestBed, fakeAsync, tick } from '@angular/core/testing';
+import { Toast, ToastService } from './toast.service';
+
+describe('ToastService', () => {
+  let service: ToastService;
+
+  beforeEach(() => {
+    TestBed.configureTestingModule({});
+    service = TestBed.inject(ToastService);
+  });
+
+  it('show() emits a toast with correct message and type', (done) => {
+    service.current$.subscribe((toast) => {
+      if (toast) {
+        expect(toast.message).toBe('Hello!');
+        expect(toast.type).toBe('error');
+        done();
+      }
+    });
+    service.show('Hello!', 'error');
+  });
+
+  it('type defaults to success when not provided', (done) => {
+    service.current$.subscribe((toast) => {
+      if (toast) {
+        expect(toast.type).toBe('success');
+        done();
+      }
+    });
+    service.show('Default type');
+  });
+
+  it('current$ emits null after the duration elapses', fakeAsync(() => {
+    const emitted: (string | null)[] = [];
+    service.current$.subscribe((t: Toast | null) => emitted.push(t ? t.message : null));
+    service.show('Auto-dismiss', 'success', 500);
+    tick(500);
+    expect(emitted).toContain(null);
+  }));
+
+  it('calling show() again before timer fires replaces the active toast', fakeAsync(() => {
+    const messages: (string | null)[] = [];
+    service.current$.subscribe((t: Toast | null) => messages.push(t ? t.message : null));
+    service.show('First', 'success', 1000);
+    tick(200);
+    service.show('Second', 'success', 1000);
+    tick(0);
+    const nonNull = messages.filter((m) => m !== null);
+    expect(nonNull[nonNull.length - 1]).toBe('Second');
+  }));
+});